I have the same problem, it seems to be a bug.
My workaround is to do it via command line CCO command:
Share is on my QNAP \192.168.100.20\Media, Subfolder are Files\I-Tunes
Share should be acessible only to a group Audio:
sudo -u www-data php ./occ files_external:create AUDIO smb password::password -c host=192.168.100.20 -c share=/Media/ -c root=/Files/I-Tunes -c user=username -c password='Password'
--> Will return a ID for the Share, for example 9
sudo -u www-data php ./occ files_external:applicable --add-group Audio 9
Works perfekt as workaround